1. Field of the Invention
The present invention relates to a sheet feeding apparatus used for an image forming apparatus such as a copying machine and a printer.
2. Description of the Related Art
An image forming apparatus which forms an image on a sheet is generally provided with a sheet feeding apparatus for continuously feeding sheets to an image forming unit included in the image forming apparatus. The sheet feeding apparatus includes a motor-driven elevatable tray for stacking sheets, and side regulating members and a trailing edge regulating member for positioning sheets on the tray. The sheet feeding apparatus also includes a pickup roller for feeding an uppermost sheet stacked on the tray and a separation unit for separating from other sheets the sheet fed from the pickup roller.
To maintain the uppermost sheet stacked on the tray to an appropriate position for feeding with the pickup roller, the sheet feeding apparatus also includes a sheet surface detection unit for detecting the position of the sheet top surface. The sheet feeding apparatus also includes a control unit for controlling a drive unit such as a motor to set the uppermost sheet to vertically move the tray so that the uppermost sheet comes to an appropriate position in response to the sheet surface detection by the sheet surface detection unit. In general, the sheet surface detection unit includes a sensor lever rotatably disposed above the tray and a sensor (such as a photo-interrupter) which turns on or off when the sensor lever is pressed by the sheet top surface and then rotated.
In addition to the sheet surface detection unit, the sheet feeding apparatus also includes above the tray a sheet presence detection unit for detecting the presence of a sheet stacked on the tray. The sheet presence detection unit has a similar configuration to the sheet surface detection unit. When the sheet runs out, the sensor lever falls into an opening provided at a position on the tray corresponding to the sensor lever, thus the sheet presence detection unit detecting the absence of sheet.
Generally, an elevating mechanism for vertically moving the tray supports the tray with a wire. The motor winds and rewinds the wire to vertically move the tray, respectively. At the time of shipment and delivery to a user, such a sheet feeding apparatus is packed together with an image forming apparatus before transportation. In this case, the tray in the sheet feeding apparatus is suspended by a wire and, therefore, is unstable.
Conventionally, the tray is fixed by padding packing material such as corrugated paper to prevent damage to the tray. Japanese Patent Application Laid-Open No. 06-282134 and Japanese Patent Application Laid-Open No. 02-108883 discuss such a technique.
For example, when the power of the image forming apparatus is turned on with the packing material failed to be removed from the sheet feeding apparatus, the motor of the elevating mechanism starts but the tray cannot be vertically moved since it is fixed by the packing material. Therefore, excessive load is applied to the elevating mechanism, possibly causing damage to gears for transmitting the driving force from the motor. Problems other than the above-mentioned ones will be described below with reference to an example of the conventional packing material.
As illustrated in FIG. 10A, a feeding deck (component of the sheet feeding apparatus) includes a tray 51, side regulating members 65a and 65b for regulating the side ends of sheets stacked on the tray 51, and a trailing edge regulating member 66 for regulating the trailing edge thereof. The feeding deck also includes a pickup roller 16a for feeding the uppermost sheet of the sheets stacked on the tray 51, and a pair of a feeding roller 16b and a retarding roller 16c for separating from other sheets the sheet fed from the pickup roller 16a. To control the vertical motion of the tray 51, the feeding deck also includes the sheet surface detection unit for detecting the top surface position of the stacked sheet and the sheet presence detection unit for detecting the presence of sheet on the tray.
The sheet surface detection unit includes sheet surface detection sensors 53 and 54, which generate respective ON/OFF signals based on the position of the pickup roller 16a. A holder 60 which supports the pickup roller 16a is provided with the function of a detection lever. The holder 60 has flags formed thereon for causing the sheet surface detection sensors 53 and 54 to generate the ON/OFF signals. Then, the ON/OFF signals of the sheet surface detection sensors 53 and 54 are generated by the flags of the holder 60 in relation to the position of the pickup roller 16a. The position of the sheet top surface is detected by the control unit based on the ON/OFF signals. More specifically, when the tray 51 rises, the sheet top surface pushes up the pickup roller 16a. When the ON/OFF signals of the sheet surface detection sensors 53 and 54 are generated by the flags, the position of the sheet top surface is detected.
The sheet presence detection unit includes a detection lever 52 and a sheet presence detection sensor 61. When the detection lever 52 falls into an opening (not illustrated) formed on the tray 51, the absence of sheet is detected.
When a conventional packing material 71 having a concaved central part is used, as illustrated in FIG. 10A, when the power of the image forming apparatus is turned on with the packing material 71 failed to be removed, the sheet surface detection sensors 53 and 54 determine that the sheet top surface S is low. Therefore, the elevating mechanism (not illustrated) raises the tray 51, resulting in a state illustrated in FIG. 10B. In this case, before the packing material 71 is detected by the sheet presence detection sensor 61 or the sheet surface detection sensors 53 and 54, a top surface 71a of the packing material 71 contacts the detection lever 52 of the sheet presence detection unit, thus causing damage thereto.
When a packing material 72 having a rectangular parallelepiped shape as illustrated in FIG. 11A is used as another conventional packing material, failure to remove the packing material 72 causes the following problems.
When the power of the image forming apparatus is turned on, the sheet surface detection sensors 53 and 54 determine that the position of the sheet S top surface is low. Therefore, the elevating mechanism (not illustrated) raises the tray 51. In this case, the packing material 72 causes the same state as the one where the sheet S is stacked on the tray 51 of the sheet feeding apparatus. Therefore, the control unit incorrectly recognizes the presence of sheets based on the sheet presence detection by the sheet presence detection sensor 61, and performs the sheet feed operation. More specifically, the tray 51 is raised until the top surface of the packing material 72 reaches a predetermined height at which the sheet feed operation by the pickup roller 16a is started. When the pickup roller 16a performs the sheet feed operation in this state, as illustrated in FIG. 11B, the packing material 72 and the pickup roller 16a are in friction, causing such damage as abrasion to the surface of the pickup roller 16a. 
In recent years, since an increasing number of image forming apparatuses have performed image formation on a large amount of diverse types of sheets, it has become necessary to prepare a number of sheet feeding apparatuses for one image forming apparatus. Accordingly, a failure to remove the packing material is likely to occur.